Nickel plating by chemical reduction



Patentecl Dec. 5, 1950 mcxm. PLATING BY o'nsmoan naouc'rron Abner Brenner, Chevy Chase, Md., and Grace E. Riddell, Washington, D. C.

No Drawing. Application May 5, 1947,

Serial No. 746,151

11 Cillml. (Cl. 117-40) (Granted under the act of March a, 1883, as amended April so, 1928; m o. o. m)

The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to us of any royalty therei on in accordance with the provisions of the act of April 30, 1928 (ch. 460, 45 Stat. L. 467).

This invention relates to plating by chemical reduction, and more particularly to a method and bath for plating metallic surfaces with a coating of nickel by autocatalytic chemical reduction reaction.

For a statement relative to certain phases of this invention and also to the prior art, attention is directed to the article Nickel Plating on Steel by Chemical Reduction by Abner Brenner and Grace E. Riddell, at pages 31 to 34, Volume 37, July 1946 issue, Journal of Research of the National Bureau of Standards.

Heretofore, it has been common practice to plate metallic surfaces with nickel by electrodeposition. The electrolytic process, however, requires the use of special and relatively expensive equipment, such as generators, rheostats, and racks. Thus, the electrolytic process is impractical when such special equipment is not at hand or its cost is prohibitive.

It has also heretofore been proposed to produce metallic nickel by the reduction of nickel solutions with hypophosphite. Such prior procedure however encountered disadvantages which discouraged its adoption and practice. The prior proposal was the use of high concentrations of hypophosphite usually several hundred grams per litenof solution, to effect the reduction. The reaction in the reduction process was usually quite vigorous, much hydrogen was evolved, and the mass bubbled up to or times its original volume.' The product of the reduction was mainly a dark powder, but occasionally it also deposited on the walls of the reaction vessel as a coating which gradually became detached and broke up into thin flakes.

The present invention is the discovery of an autocatalytic chemical reduction process and bath for plating metallic surfaces with nickel. The present process has been found to provide selective deposition of the plating metal, no appreciable metal being precipitated to the bottom of the reaction vessel or being deposited on its walls. The plating metal is deposited in substantially pure, metallic form, has good adhesion to the plated metallic surfaces, and in salt-spray tests has been found to have the same protective value as the plate secured by electrodeposition.

According to the present invention. the source of the plating material or metallic coating is a nickel salt. The metallic object is immersed in the plating bath until it has a plating deposit or coating of desired thickness. The bath is an aqueous solution of the nickel salt and contains a relatively low concentration of hypophosphite.

Using sodium hypophosphite as a typical hypophosphite, the reaction of the present invention by which nickel is produced is Equation 1 or Concurrently, some of the hypophosphite is oxidized by the water, particularly in the presence of certain metals, to phosphite, and hydrogen is liberated:

- NaHzPOz H20- NaH2PO3 +H2 Equation 2 Equations 1 and'2 show that the reaction mixture becomes more acid as either an acid salt or free acid is produced. The reduction of nickel ion (Eq. 1) is catalyzed by certain metals, including .nickel, and as nickel is produced by the reaction it is therefore autocatalytic. This explains why the reaction, which is rather slow in starting, proceeds with so much vigor after it once begins.

It is therefore an object of this invention to provide a novel and relatively simple chemical reductionprocess for plating metallic surfaces with nickel.

Another object is to provide an autocatalytic chemical reduction process for the selective depositibn of nickel in the plating of metallic surfaces.

A further object is to provide a plating process by which a relatively hard, adherent and protective coating is deposited on metallic surfaces.

Another object is to provide an autocatalytic chemical reduction process for plating metallic surfaces, which process requires only materials that are readily available and obtainable.

Stillanother object is to provide a. plating process which eliminates the need for special equipment such as is required in electrodeposition.

Other objects and advantages of this invention will be apparent from the following description and the appended claims. r

In general, this process is applicable to the plating of nickel on a large group of metals, such as steel. iron, platinum, silver, nickel, gold, co-

assaaas balt, palladium, aluminum, copper or copper base alloys. In advance of the plating treatment, the metallic objects, which are to have their surfaces plated, are preferably cleaned by any of the accepted and conventional procedures and then given a conventional acid dip. Objects of steel,

im platinum, silver, nickel, gold, cobalt, palladium and aluminum having been cleaned and acid-dipped are ready for immersion in the plating bath.

Objects of copper and copper base alloys, including brass and bronze, should be given additional pre-plating treatment in the form of a momentary bright acid dip followed by a dip for about one minute in a solution of 0.02 gram of palladium chloride (PdClz) and 20.0 grams of hydrochloric acid per liter of solution. The concentration and length of the palladium dip can be varied inversely. and when the solution is heated, can be decreased.

Objects of platinum and silver, when immersed in the plating bath require momentary contact with aluminum to initiate the plating operation.

The objects to be plated may be suspended in the plating bath by a string. Small objects may be supported by resting them on cloth stretched over a frame immersed in the plating bath. Small objects while in the bath should be agitated occasionally, although there is no need of constant motion, as in barrel plating, because electric current distribution is not involved.

In proceeding to a description of the bath composition, it should be noted that wherever reference is hereafter made to parts by weight, it is to be understood that the amount is given in parts by weight per 100 parts by weight of the plating bath.

The plating bath or solution of this invention comprises: Parts by weight Nickel ion of a nickel salt 0.05 to Hypophosphite radical 0.0! to 7.5

Water 50 to98 The nickel ion is provided by the use of such water soluble nickel salts as nickel acetate, nickel chloride, and/or nickel sulfate.

The hypophosphite radical in the plating bath may be secured by the use of sodium hypophosphite. ammonim hypophosphite, or potassium hypophosphite.

Throughout the plating process, the bath according to a preferred embodiment of this invention is heated to maintain it at a temperature in the range of about 90 C. to about 100 C. The boiling temperature of the bath, however, is to be avoided. The indicated range of temperature has been found to promote the chemical reduction reaction. At lower temperatures the reaction proceeds slowly and so does the deposition of the plating metal. At higher temperatures than indicated, the bath evaporates, and in the case of the ammoniacal bath, the ammonia escapes to an undesirable degree. The temperature of the bath during the plating operation may be any temperature in the range of about C. to about 100 C., but it is found that the rate of deposition is greatest and the soundness of the deposits is better if the temperature is maintained in the 90 C. to 100 0. range.

The hypophosphite baths of this invention are fairly stable. At room temperature, the bath is stable for several weeks. At 90 C., the bath over a period of as much as 5 hours shows negligible deterioration.

The thickness of the plating deposit in a given length of time depends in part on the relation of the area of the surface of the metal object being plated to the total volume of the plating bath composition. In general, the rate of plating is somewhat less than that usually employed in electroplating with the same metals in a tank, but is about the same as in barrel plating. Because of the gradual exhaustion of the hypophosphite, less deposit is secured during each successive unit interval of time. Usually the rate of deposition is greatest during the first hour, and the reaction is virtually complete in two hours, unless additional hypophosphite is introduced into the bath. Some of the typical rates of deposition per hour are stated hereinafter.

As the hypophosphite is gradually exhausted, more hypophosphite must be added if thick deposits are required, especially if the area to be plated is relatively large compared to the volume of the plating solution being used. Additions of five grams of hypophosphite per liter of bath solution may be made at half-hour to one-hour intervals. Thereby, deposits 0.002 inch (0.05 mm.) thick have been obtained in about six hours. As pointed out in Equations 1 and 2, the hypophosphite becomes converted into phosphite, which however does not interfere with the operation of the bath. When the bath becomes too concentrated with the phosphite, it is more economical to discard the bath than to try to remove the phosphite.

Under the conditions provided by this invention, the plating is selective: that is, the plating occurs only on the surface of the metal objectnot on the walls of the bath vessel.

After a bath has operated in a glass vessel for about five hours. a small amount of a precipitate containing the pla ing metal may deposit on the bottom of the bath ve el where the source of heat has been applied. If this metallic precipitate is not removed. it may de ompo e some of the hypophos hite, just as another metal surface does. The removal can be ecured by filtering.

The yield of lating metal u ually ranges between 20 and 37 per cent. ba ed on the conversion of the h po ho phite to pho hite. The 37 per cent y eld has been obta ned with a lar e area of metal urfa e be n plated in he hath and with a reduction of about 2 rams of plat n metal by 10 rams of the hypopho phite salt. With a relati el small area of metal surface bein plated, the deposition or yield m be about 20 per cent. H her yields are not obtained as some of the hvno ho h te i catalvtinallv oxidized b water w th the li eration of h dro en. The o t mum yields are decrea ed by the pre ence of su h ions as cadmium. copper. z nc. magne ium. and thincyana e. The e contaminants can be remo ed by conventional procedures. and the rate of deposition brou ht back to normal.

Tests of products resulting from the present process show that the plating depo it has good adhesion to the ba e metal and that the adhesion may be improved by acid-dip treatment of the cleaned base metal prior to the plating o eration. Tests with a twenty per cent salt spray also show that nickel plated steel produced by this invention has a protective value comparable to nickel plated steel produced by electrodeposition and having a nickel plating of the same thickness.

pH VALUE OF BATH OR PLATING SQLUTION lit has been discovered that the plating process m y be successfully performed by maintaining s the pH value of the bath in the range of about two (2) to about eleven (11).

In practicing this invention with an alkaline bath (pH value of about 7 to about 11) the nickel and hypophosphite salts are used for the described purposes and other compounds are used to control the pH- value and to hold the nickel ion in solution. If the solution consists only of the nickel and hypophosphite salts, water, and material for holding the nickel salt in solution, it initially has a,ssa,aes

a slightly alkaline pH value, but proceeds rapidly to an acid pH value and becomes less eilicient. However, it is found that the present method may be practiced with a plating solution having an acid pH value (pH value of about 2 to about 7), if the solution is provided with a salt of carboxylic acids which afford relatively high rates of plating deposition in a bath of acid pH value.

a Organic salt 0.00 to 40.0 Ammonium salt 0.00 to 10.0

The limits for the salts represent the possible ranges when one of them is varied at one time.

When the bath of this invention is operated with a pH value of about 2 to about 7, the acidity is maintained and the nickel salt is held in solution preferably by the use of a sodium salt of a weak organic acid, such as sodium acetate, or sodium salts of hydroxycarboxylic acids, such as sodium citrate, and sodium hydroxyacetate.

These organic salts act as buflers (pH regulawhen the bath of this invention is operated with a pH value of about 7 to about 11, the alkalinity is secured by the use of such basic compounds as ammonium hydroxide, and the nickel salt is held in solution by the use of sodium citrate, tartaric acid or other salt of a hydroxy organic acid and/or ammonium salts. The addition of ammonia water (NH4OH), or other hy- Nickel ions; 0.05 m 10.0 Hypophosphitc radical 0.07 to, 7.5

tors) in the acid solution. The acidity may also be maintained by the use of hydroxides of ammonium, potassium or sodium. These hydroxides may replace the organic salts or may be used with them to control the acidity of the solution. Experience shows that the preferred condition for deposition is one in which the pH value is maintained within relativelynarrow limits throughout the plating operation. Thus, it is recommended that the pH value he maintained within the range of about 3 to 6 if an acid bath is used. The pH value is readily controlled by the addition of a hydroxide to the bath and during the plating operation.

For an acid bath of this invention, the range of composition in parts by weight per 100 parts by weight of solution is as follows:

Nickel ions 0.05 to 10.0 Hypophosphite radical 0.07! to 7.5 Organic salt 0.00 to 25.0

EXAMPLES The following tables show eramples oi plating baths or solutions embodying this invention:

Or to point of saturation.

Table L-Bath compositions exemplary of the alkaline range I II III Nickel Chloride (NiCl:.6HzO) 3 3 3 Sodium Hypophosphite (NBHQPOLHQO)- 1 l 1 Ammonium Chloride (NH4C1) 5 l0 Sodium Citrate 2 (Nui0sHnO ).11H:O).-... 10 l0 Watcr (H1O) 81 86 86 Total 100 100 100 Rate of deposition:

mmJhr 0.006 0.012 0.004 inches/hour 0. 00025 0. 0005 0. 00015 Appearance of deposit Semi- Dull Semi- Bright Bright Alkali for neutralizing NH4OH NH40H N 210}! P 5-9 7-0 Table L-Bath compositions of the alkaline range-continued IV V VI VII VIII Nickel Chloride 10 3 i 3 3 NickelAcetate Ni(C;H;02).4H10 3 Sodium Hypophosphite. 1 10 l 2' i Ammonium Chloride 5 5 5 5 Sodium Citrate l0 l0 10 40 Sodium Hydroxyacetate. l0 Water 74 72 86 51 Total 100 100 100 100 Rate oi deposition:

mm./hr .0056 025 0. 011 0. 011 0.005

0 in. r .00022 .00 0.0005 0. 0.00025 Appearance of deposit bright semibright dul bright bright A kali for neutralizing NH40H HNiO N H4011 NH4OE NHOH vcentration shows no appreciable change in the Table II.Bath compositions exemplary of the acid range III Nickel Chloride (NiClz.6H).....-.. Nickel Sulfate (NiSOIJHflO) Sodium Acetate Naclnaoaanlon... Sodium Citrate 2(NaaOsH 0 ).l1HzO) In the foregoing tables the compositions are expressed in parts by weight; the rate of deposition is shown in millimeters per hour and also in inches per hour; the appearance of the deposits is set out; and the pH value for each example is shown. Table I also shows a suitable neutralizing agent.

The bath compositions set out in the foregoing tables are useful in plating objects of any of the metals heretofore described. The soluble nickel salts serve as the source of the nickel ions, and the hypophosphite serves as the reducing agent.

Th organic salts in the alkaline baths serve to hold the nickel salts in solution, and in the acid baths, serve as buffers to regulate the pH value.

In an alkaline bath the organic salt is preferably sodium citrate. In an acid bath, sodium acetate and sodium hydroxyacetate are the preferred organic salts.

In the alkaline baths, ammonium salts serve to maintain the alkalinity and to hold the nickel salt in solution.

It is found that an increase in the nickel ion concentration in the bath from 0.05 to 1.0 part by weight results in an increase in the rate of deposition, and that further increase in conrate of deposition.

In an alkaline bath, the rate of deposition is proportional to the concentration of hypophosphite to a substantial extent. However, in the acid bath, the rate of deposition is affected very little by hypophosphite additions which increase the hypophosphite radical concentration beyond two parts by weight per 100 parts of solution.

As to the use of the organic salts, it is found that high concentrations decrease materially the rate of deposition since the nickel ions enter into complex formations. In alkaline baths having low concentration of the organic salt, the nickel salts are not held completely in solution. In acid baths having a low concentration of the organic Water (H 0) 91 95.1 95

Total 160 100 100 Rate oi Deposition:

mm. our .012 .012 .005 .02) iDChGS/hfliflli.) W2..- 8 R e em o g Appearanc o eposi Bright Dun, pH 3-6 Table II.--Bath compositions of the acid range-continued V VI VII Nickel Chloride 15 .4 3 Sodium Hypophosphite i l 10 Sodium Hydroxyacetate. 5 5 Water 84 93.6 82

0. 0005 0.006 0. 015 .00002 .00024 .0006 Dullirregular Dull Dull 4-6 46 4-6 salt, the organic salts do not act efficiently as buffers for providing an optimum rate of deposition.

When organic salts are used in the bath, it has been found that lowering of the organic salt concentration results in a higher rate of deposition and a duller deposit, whereas the reverse is true when higher concentrations are used.

In the alkaline baths, it is found that a lesser amount of the organic salt is required if the concentration of the ammonium salt is increased. It is also found that the ammonium salt may be replaced by sodium hydroxide to furnish the required pH value, but the control of such a bath is more difiicult.

The foregoing is to be understood as illustrative as this invention includes all modifications and embodiments coming within the scope of the appended claims.

We claim:

1. An autocatalytic chemical reduction process for continuously plating nickel on metallic objects consisting essentially of a metal selected from the group consisting of iron, platinum, silver, nickel, gold, cobalt, palladium, aluminum, and copper, said process comprising the steps of: providing an aqueous solution of a nickel salt and an alkaline hypophosphite, the nickel ion being present in an amount not substantially in excess of about 3 parts by weight to about 100 parts by weight of said solution, the hypophosphite radical being present in an amount not substantially in excess of about 3 parts by weight to about 100 parts by weight of said solution, immersing an object of one of said metals in said solution, nickel plating said immersed object by autocatalytic reaction of said salt and said hypophosphite, removing said plated object from said solution, maintaining said solution throughout the period of said immersion at a temperature in the range of about 90 degrees centigrade to about 100 degrees centigrade and at a substantially uniform pH value in the range of about 3 to about 9 by the presence in the solution of at least one of the salts selected from the group consisting of sodium salts of carboxyllc acids and ammonium salts. and introducing throughout said immersion period and at substantially regular intervals additional hypophosphite into said solution in amounts of about 5 grams per liter of said solution.

2. A process as defined in claim 1 wherein the object is platinum and when immersed is momentarily contacted with aluminum to initiate the autocatalytic reaction.

3. A process as defined in claim 1 wherein the object is silver and when immersed is momentarily contacted with aluminum to initiate the autocatalytic reaction.

4. A process as defined in claim 1 wherein the metallic object consists essentially of copper and prior to immersion in the nickel plating solution is dipped in a solution of palladium chloride and hydrochloric acid.

5. A process as defined in claim 1 wherein the plating solution is provided with a sodium salt of a carboxylic acid and wherein the pH value of the plating solution throughout the immersion period is maintained within the range of about 3 to about 6 by the use of a basic hydroxide solution.

6. A process as defined in claim 1 wherein the last named salt is a sodium salt of a hydroxy carboxylic acid and wherein the pH value of the plating solution throughout the immersion period is maintained within the range of about 8 to about 9 by the use of a dilute basic hydroxide solution.

7. An autocatalytic chemical reduction process for continuously plating nickel on metallic objects consisting essentially of a metal selected from the group consisting of iron, platinum, silver, nickel, gold, cobalt, palladium, aluminum,

and copper, said process comprising the steps of:

providing an aqueous solution of 3 parts by weight of nickel chloride, 1 part by weight of sodium hypophosphite, 5 parts by weight of ammonium chloride and 10 parts by weight of sodium citrate per 100 parts by weight of solution, immersing an object of one of said metals in said solution, nickel plating said immersed object by autocatalytic reaction or said nickel salt and said hypophosphite, removing said plated object from said solution, maintaining said solution throughout the period of said immersion at a temperature in the range of about 90 C. to about 100 C. and at a pH value in the range of about 8 to about 9 by the addition of ammonium hydroxide, and introducing additional hypophosphite into said solution and at substantially regular intervals in amounts of about 5 grams per liter of said solution.

8. An autocatalytic chemical reduction process for continuously plating nickel on metallic objects consisting essentially of a metal selected from the group consisting of iron, platinum, silver, nickel, gold, cobalt, palladium, aluminum, and copper, said process comprising the steps of: providing an aqueous solution of 3 parts by weight 01' nickel chloride, 1 part by weight of sodium hypophosphite, and 5 parts by weight of sodium hydroxyacetate per 100 parts by weight of solution, immersing an object of one of said metals in said solution, nickel plating said immersed object by autocatalytio reaction of said nickel salt and said hypophosphite, removing said plated object from said solution, maintaining said solution throughout the period of said immersion at a temperature in the range of about C. to about C. and at a pH value in the range of about 4 to about 6 by the addition of soluble basic hydroxide, and introducing additional hypophosphite into said solution and at. substantially regular intervals in amounts of about 5 grams per liter of said solution.

9. A process as defined in claim 8 wherein the object is platinum and when immersed is momentarily contacted with aluminum to initiate the autocatalytic reaction.

10. A process as defined in claim 8 wherein the object is silver and when immersed is momentarily contacted with aluminum to initiate the autocatalytic reaction.

11. A process as defined in claim 8 wherein the metallic object consists essentially of copper and prior to immersion in the nickel plating solution is dipped in a solution of palladium chloride and hydrochloric acid.

ABNER BRENNER. GRACE E. RIDDELL REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,207,218 Roux Dec. 5, 1916 2,265,467 Alexander Dec. 9, 1941 

1. AN AUTOCATALTIC CHEMICAL REDUCTION PROCESS FOR CONTINUOUSLY PLATING NICKEL ON METALLIC OBJECTS CONSISTING ESSENTIALLY OF A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, PLATINUM, SILVER, NICKEL, GOLD, COBALT, PALLADIUM, ALUMINUM, AND COPPER, SAID PROCESS COMPRISING THE STEPS OF: PROVIDING AN AQUEOUS SOLUTION OF A NICKEL SALT AND AN ALKALINE HYPOPHOPHITE, THE NICKEL ION BEING PRESENT IN AN AMOUNT NOT SUBSTANTIALLY IN EXCESS OF ABOUT 3 PARTS BY WEIGHT TO ABOUT 100 PARTS BY WEIGHT OF SAID SOLUTION, THE HYPOPHOSPHITE RADICAL BENG PRESENT IN AN AMOUNT NOT SUBSTANTIALLY IN EXCESS OF ABOUT 3 PARTS BY WEIGHT TO ABOUT 100 PARTS BY WEIGHT OF SAID SOLUTION, IMMERSING AN OBJECT OF ONE OF SAID METALS IN SAID SOLUTION , NICKEL PLATING SAID IMMERSED OBJECT BY AUTOCATALYTIC REACTION OF SAID SALT AND SAID HYPOPHOPHITE, REMOVING SAID PLATED OBJECT FROM SAID SOLUTION, MAINTAINING SAID SOLUTION THROUGHOUT THE PERIOD OF SAID IMMERSION AT A TEMPERATURE IN THE RANGE OF ABOUT 90 DEGREES CENTIGRADE TO ABOUT 100 DEGREES CENTIGRADE AND AT A SUBSTANTIALLY UNIFORM PH VALUE INT HE RANGE OF ABOUT 3 TO ABOUT 9 BY THE PRESENCE IN THE SOLUTION OF AT LEAST ONE OF THE SALTS SELECTED FROM THE GROUP CONSISTING OF SODIUM SALTS OF CARBOXYLIC ACIDS AND AMMONIUM SALTS AND INTRODUCING THROUGHOUT SAID IMMERSION PERIOD AND AT SUBSTANTIALLY REGULAR INTERVALS ADDITIONAL HYDPOPHOPHITE INTO SAID SOLUTION IN AMOUNTS OF ABOUT 5 GRAMS PER LITER OF SAID SOLUTION. 